MS wants people to run Win10, therefore it's logical that old keys will still activate.
Yeah. It's about other aspects of their costs -- in support for Win7 and Win 8, and maintenance for Win 10. You could ask "What would happen if Win7-to-Win10 upgraders were asked to pay for a new build at some future time? Imagine what THOSE costs might be. That's why they don't go there. So they'll honor their commitment to the original Free Win 10 under Win 7 product key, and they should honor it for Anniversary through Creators Update and updates beyond that.
Here's what I think is likely as I've seen it so far. They'll let you keep using the old 1607 Build and they will continue providing updates to maintain it. If anyone knows differently about that, chime in here and let us know. Right now I'm poised to try again on my dual-boot configuration and I can now do it through updates (which had stopped working but it was an easy fix), through the download/update website where the updater takes control of your system within a session of the 1607 configuration, and through either USB or ISO installation media.
I don't see a need or reason to circumvent the "Media Creation Tool" with the EDGE F12->Emulation settings, unless you want a generic installation medium. I think with Media Creation Tool, the installation medium and content you get will be optimal for the machine on which it is made.
What is needed here or in a more directly appropriate thread is a growing set of observations about the actual comparative advantages of Build 1703.
I could be paranoid enough to think MS "planned" the obstacle thrown up by my Windows Update -- a problem eliminated by uninstalling "ghost" drive/driver installations. But to discover it, any number of users would have to use PowerShell to crack the update logs and put them in readable format, and they'd have to know what to look for. That's too paranoid. But there have been indications in articles posted on the web prior and after April 11 -- indications that people had an obstacle with Windows Update to arrive at a screen that checks and completes updates to Build 1607 while offering Build 1703 in a little two-line advertisement and link-invitation.